Mechanical-hydraulic transmission gear system and method of controlling power transmission using the system

ABSTRACT

A mechanical-hydraulic transmission gear system for use in construction vehicles or construction equipment and a method of controlling power transmission using the system arranged such that when a prime mover is rotating at high speeds, power transmission is effected only through a mechanical transmission gear to thereby reduce the power loss due to fluid pressure substantially to zero, while when the prime mover is rotating at low speeds power transmission is effected only through a hydraulic transmission gear so as to facilitate control of the speed of the vehicle and control of changing forward running over to reversing and vice versa. The mechanical-hydraulic transmission gear system comprises a hydraulic transmission gear (10) having a hydraulic pump (11) and a hydraulic motor (13); a mechanical transmission gear (30); change-over means (26) for changing forward running of the vehicle over to reversing and vice versa; a speed control means (27); a sensor means (22) for sensing the rotational speed of output shaft (14); and control means (20) for comparing the signals transmitted by the change-over means, the speed control means and the rotational speed sensor means so as to effect control of disconnection and connection of a clutch (31) mounted on an input shaft (2) of the mechanical transmission gear and control of discharge of fluid from the pump and the motor. The method of controlling power transmission using this system includes the steps of disconnecting the clutch of the mechanical transmission so as to transmit the power from the prime mover only through the hydraulic transmission gear to the output shaft when the rotational speed of the output shaft is lower than a predetermined value; and connecting the clutch of the mechanical transmission gear so as to transmit the power only through the mechanical transmission gear to the output shaft and minimizing the power required by the hydraulic transmission gear according to a command from the control means when the rotational speed of the output shaft is higher than the predetermined value.

TECHNICAL FIELD OF THE INVENTION

This invention relates to a mechanical-hydraulic transmission gearsystem and a method of controlling power transmission using the gearsystem, more particularly this invention relates to a power transmissionsystem for use in construction vehicles or construction equipment suchas wheel type power shovels and rough terrain cranes, etc and a methodof controlling power transmission using the gear system, wherein whenthe prime mover thereof is rotating at high speeds, the power loss dueto fluid pressure is reduced substantially to zero so as to avoid alowering in power transmission efficiency, and power transmission iseffected only through the mechanical transmission gear so as to improvetractive force. When the prime mover is rotating at low speeds, speedcontrol and control of change-over from forward rotation to reversingand vice versa can be made readily, and power transmission is effectedonly through the hydraulic transmission gear so as to improve thecomposite maneuverability of implement.

BACKGROUND ART OF THE INVENTION

As the power transmission gear of construction equipment, etc. thefollowing systems have heretofore been used.

1. Mechanical power transmission system

2. Hydraulic power transmission system (HST)

3. Mechanical-hydraulic power transmission system (HMT)

In the mechanical-hydraulic power transmission system (HMT), part of thepower is transmitted mechanically and the rest of the power istransmitted hydraulically; in other words, a power division typehydraulic transmission system (hydraulic-mechanical power transmission),among which an output division type as shown in FIG. 1 is well known.

However, in the above-mentioned prior art transmission gears, themechanical power transmission system is excellent in power transmissionefficiency, but poor in the control of change-over from forward runningto reversing and vice versa. The hydraulic power transmission system(Refer to FIG. 2) is capable of effecting control of change-over fromforward rotation to reversing and vice versa, by varying the tiltingangle of swash plates of a variable displacement hydraulic pump 51 and avariable displacement hydraulic motor 52. The system and therefore canbe operated easily, and is also capable of changing the rotational speedof the output shaft by engagement between gears 53 and 54, engagementbetween gears 55 and 56 and change-over between clutches 57 and 58.However, such a system is disadvantageous in that when the output shaftis rotating at high speeds the tilting angle of the swash plate of thepump 51 is increased to increase the discharge flow-rate of fluid thusincreasing the pressure loss, while the tilting angle of the swash plateof the motor 52 is reduced thus causing a lowering in efficiency. Forthis reason, when the output shaft of the mechanical-hydraulic powertransmission system (Refer to FIG. 1) is rotating at high speedsmechanical power transmission is effected so as to avoid lowering inefficiency. While when the output shaft is rotating at low speeds,hydraulic power transmission is effected so as to facilitate control ofchange-over from forward rotation to reversing and vice versa. This typeof system has been used in various fields.

In FIG. 1, an input shaft 62 is connected to a prime mover 61, and anintermediate portion of the input shaft 62 has a gear 64 fixedly securedthereto and which meshes with a hydraulic pump driving gear 63. Further,a sun gear 65 of a planetary gear device A is fixedly secured to one endof the input shaft 62. The hydraulic pump driving gear 63 is arranged todrive a variable displacement type pump 66 of a hydraulic transmissiongear B provided on one side of the input shaft of driving gear 63. Thisvariable displacement type pump 66 is arranged to actuate a fixeddisplacement motor 67 mounted in juxtaposition thereto and parallel withthe axis of the input shaft 62. This fixed displacement motor 67 has anoutput shaft provided with a gear 68 to which the power developed by themotor 67 is transmitted. The gear 68 is arranged to rotatively drive aninternal gear 69-1 of the planetary gear device A through a gear 69which meshes therewith. A planetary gear 70 is provided between theinternal gear 69-1 and the sun gear 65. The planetary gear 70 has ashaft supporting frame 71 which is connected to an output shaft 72. Thearrangement is made such that revolution of the planetary gear 70 aroundthe sun gear 65; that is, rotational motion of the shaft supportingframe 71 can be transmitted to a starting wheel 73 mounted on the bodyof a construction vehicle. When the variable displacement type pump 66is at its neutral position, the hydraulic transmission gear B conductsonly braking action and the internal gear 69-1 is fixed. Therefore theplanetary gear device A serves as a mechanical planetary reduction gear.Speed control of the output shaft 72 is effected by a speed controlmeans, not shown, such as a governor or the like provided in the primemover 61. Further, when the variable displacement type pump 66 isactuated, part of the power developed by the prime mover is transmittedin turn through transmission elements 64, 63, 66, 67, 68, 69 to theplanetary gear 70, while the rest of the power is mechanicallytransmitted in turn through transmission elements 62, 65 to theplanetary gear 70.

The above-mentioned prior art mechanical-hydraulic power transmissionsystem has the following disadvantages. Since the fixed displacementmotor 67 is arranged to drive the internal gear 69-1, in order to changethe number of revolutions of the output shaft from forward rotation toreversing, it is essential to use a double-discharge, variabledisplacement pump 66 as the hydraulic pump and to rotate the internalgear 69-1 at a considerably high speed when the number of revolutions ofthe output shaft 72 has reached a predetermined value. Therefore, theratio of displacement between the hydraulic motor and the hydraulic pumpmust be set at a very big value. In practice, selection of a hydraulicpump and a hydraulic motor which meet such conditions is extremelydifficult. Further, such a prior art mechanical-hydraulic transmissiongear system is disadvantageous in that, since the hydraulic pump, thehydraulic motor and the planetary gear device constituting the hydraulictransmission gear B are juxtaposed in the axial direction of the inputshaft 62, the whole system becomes large in size. Further, in the casewhere the fluid circuit is changed over to supply the fluid underpressure discharged by the hydraulic pump to another hydraulic actuatorsuch as an implement of the construction equipment, the power is alwayspartially transmitted to the output shaft 72. Even if the output shaft72 is fixedly secured by means of a brake or the like, the fixeddisplacement type motor 67 is rotated by the power transmitted from theplanetary gear 70 through the internal gear 69-1 and the gear 68thereto, thus causing rotational losses or power losses.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above-mentionedcircumstances in the prior art.

A first object of the present invention is to provide amechanical-hydraulic transmission gear system, and a method ofcontrolling power transmission using the system, wherein when the primemover thereof is rotating at high speeds, only mechanical powertransmission is effected. This reduces the power loss due to fluidpressure substantially to zero, avoids a lowering in power transmissionefficiency, and improves the tractive force. When the prime mover isrotating at low speeds, only hydraulic power transmission is effected soas to facilitate speed control and control of changeover from forwardrotation to reversing, and vice versa, and to improve the complexmaneuverability of the implement.

Another object of the present invention is to provide amechanical-hydraulic transmission gear system, and method, which enablesefficient and smooth power transmission to be effected without using aplanetary gear device, which has a simple and compact construction, andcan be manufactured at a low cost as well.

A further object of the present invention is to provide amechanical-hydraulic transmission gear system, and a method ofcontrolling power transmission using the system, wherein when fluidunder pressure discharged by a hydraulic pump is supplied to anotherhydraulic actuator by changing over the hydraulic circuit of thehydraulic transmission gear, the energy of fluid pressure can beutilized effectively without extra power loss.

A still further object of the present invention is to provide amechanical-hydraulic transmission gear system, and a method ofcontrolling power transmission using the system, wherein change-overoperations between the hydraulic transmission gear and the mechanicaltransmission gear, as well as change-over operations in each of thehydraulic transmission gear and the mechanical transmission, can beconducted smoothly and rapidly. Such is accomplished detecting changesin the rotational speed of the output shaft of the prime mover thereof,the degree of opening of the throttle, and the shift position of speedchange lever and controlling the fluid pressure to be supplied to eachclutch by means of a control unit.

To achieve the above-mentioned objects, according to a first aspect ofthe present invention, there is provided a method of controlling powertransmission using a mechanical-hydraulic power transmission gear systemincluding a prime mover having a speed control means; a hydraulictransmission gear having a variable displacement hydraulic pump and avariable displacement hydraulic motor; a control unit; and a mechanicaltransmission gear. When the rotational speed of an output shaft is lowerthan a predetermined value, the method includes the steps ofdisconnecting a clutch of the mechanical transmission gear fixedlysecured to an input shaft and transmitting the power from the primemover through the hydraulic transmission gear to the output shaftaccording to a signal transmitted by said control unit. When therotational speed of the output shaft is higher than the predeterminedvalue, the method includes the steps of connecting the clutch of themechanical transmission gear to thereby transmit the power from theprime mover through the mechanical transmission gear to the output shaftaccording to a signal transmitted by the control unit, and minimizingthe power required by the hydraulic transmission gear according to acommand from the control unit.

To achieve the above-mentioned objects, according to a second aspect ofthe present invention, there is provided a method of controlling powertransmission using a mechanical-hydraulic transmission gear system asset forth in the first aspect, characterized in that to minimize thepower required by the hydraulic transmission gear, the discharge offluid from the hydraulic pump and from the hydraulic motor are reducedto zero by actuating displacement control devices of the pump and themotor according to a command from the control unit.

To achieve the above-mentioned objects, according to a third aspect ofthe present invention, there is provided a method of controlling powertransmission using a mechanical-hydraulic transmission gear systemincluding a prime mover having a speed control means; a hydraulictransmission gear having a variable displacement hydraulic pump and avariable displacement hydraulic motor; a control unit; and a mechanicaltransmission gear, characterized in that, when the rotational speed ofan output shaft is lower than a predetermined value, the method includesthe steps of disconnecting a clutch of the mechanical transmission gearfixedly secured to an input shaft and also connecting a clutch of thehydraulic transmission gear to thereby transmit the power from the primemover through the hydraulic transmission gear to the output shaftaccording to a signal transmitted by the control unit. When therotational speed of the output shaft is higher than the predeterminedvalue, the method includes the steps of connecting the clutch of themechancial transmission gear and also disconnecting the clutch of thehydraulic transmission gear to thereby transmit the power from the primemover through the mechanical transmission gear to the output shaftaccording to a signal transmitted by the control unit, and minimizingthe power required by the hydraulic transmission gear according to acommand from the control unit.

To achieve the above-mentioned objects, according to a fourth aspect ofthe present invention, there is provided a method of controlling powertransmission using a mechanical-hydraulic transmission gear system asset forth in the third aspect, characterized in that to minimize thepower required by the hydraulic transmission gear, the discharge offluid from the hydraulic pump and from the hydraulic motor are reducedto zero by actuating displacement control devices of the pump and themotor according to a command from the control unit.

To achieve the above-mentioned objects, according to a fifth aspect ofthe present invention, there is provided a method of controlling powertransmission using a mechanical-hydraulic transmission gear system asset forth in the third aspect, characterized in that to minimized thepower required by the hydraulic transmission gear, the rotation of atleast one of the hydraulic pump and the hydraulic motor is stopped bymeans of the clutch thereof according to a command from the controlunit.

To achieve the above-mentioned objects, according to a sixth aspect ofthe present invention, there is provided a method of controlling powertransmission using a mechanical-hydraulic transmission gear systemincluding a prime mover having a speed control means; a hydraulictransmission gear having a variable displacement hydraulic pump and avariable displacement hydraulic motor; a control unit; and a mechanicaltransmission gear. When the rotational speed of an output shaft is lowerthan a predetermined value, the method includes the steps ofdisconnecting a clutch of the mechanical transmission gear fixedlysecured to an input shaft and also connecting a clutch of the hydraulictransmission gear to thereby transmit the power from the prime moverthrough the hydraulic transmission gear to the output shaft according toa signal transmitted by the control unit, and changing over therotational direction of the output shaft by actuating a hydraulic valveprovided in a fluid passage connected between the pump and the motor andadapted to be changed over according to a signal transmitted by thecontrol unit. When the rotational speed of the output shaft is higherthan the predetermined value, the method includes the steps ofconnecting the clutch of the mechanical transmission gear and alsodisconnecting the clutch of the hydraulic transmission gear to therebytransmit the power from the prime mover through the mechanicaltransmission gear to the output shaft according to a signal transmittedby the control unit, and minimizing the power required by the hyraulictransmission gear according to a command from the control unit.

To achieve the above-mentioned objects, according to a seventh aspect ofthe present invention, there is provided a method of controlling powertransmission using a mechanical-hydraulic transmission gear system asset forth in the sixth aspect, characterized in that when the hydraulicvalve is located at its neutral position and vicinity, the fluid underpressure to be supplied to the hydraulic pump can be supplied to anotherfluid circuit.

To achieve the above-mentioned objects, according to an eighth aspect ofthe present invention, there is provided a method of controlling powertransmission using a mechanical-hydraulic transmission gear system asset forth in the sixth aspect, characterized in that irrespective ofwhether or not the hydraulic valve is operated, the fluid under pressureto be supplied to the hydraulic pump can be supplied to another fluidcircuit by operating another manual fluid pressure change-over valveprovided in a fluid passage connected between the pump and the motor.

To achieve the above-mentioned objects, according to a ninth aspect ofthe present invention, there is provided a method of controlling powertransmission using a mechanical-hydraulic transmission gear systemincluding a prime mover having a speed control means; a hydraulictransmission gear having a variable displacement hydraulic pump and avariable displacement hydraulic motor; a control circuit; a mechanicaltransmission gear; and a speed increasing and decreasing device. Whenthe rotational speed of an output shaft is lower than a predeterminedvalue, the method includes the steps of disconnecting a clutch of themechanical transmission gear fixedly secured to an input shaft andtransmitting the power from the prime mover through the hydraulictransmission gear and the speed increasing and decreasing device to theoutput shaft according to a signal transmitted by the control unit. Whenthe rotational speed of the output shaft is higher than thepredetermined value, the method includes the steps of connecting theclutch of the mechanical transmission gear to thereby transmit the powerfrom the prime mover through the mechanical transmission gear and thespeed increasing and decreasing device (otherwise known as a speedmultiplying/reducing gear set) to the output shaft according to a signaltransmitted by the control unit, and minimizing the power required bysaid hyraulic transmission gear according to a command from the controlunit.

To achieve the above-mentioned objects, according to a tenth aspect ofthe present invention, there is provided a method of controlling powertransmission using a mechanical-hydraulic transmission gear system asset forth in the ninth aspect, characterized in that to minimize thepower required by the hydraulic transmission gear, the discharge offluid from the hydraulic pump and that from the hydraulic motor arereduced to zero by actuating displacement control devices of the pumpand the motor according to a command from the control unit.

To achieve the above-mentioned objects, according to an eleventh aspectof the present invention, there is provided a method of controllingpower transmission using a mechanical-hydraulic transmission gear systemincluding a prime mover having a speed control means; a hydraulictransmission gear having a variable displacement hydraulic pump and avariable displacement hydraulic motor; a control unit; a mechanicaltransmission gear; and a speed increasing and decreasing device. Whenthe rotational speed of an output shaft is lower than a predeterminedvalue, the method includes the steps of disconnecting a clutch of themechanical transmission gear fixedly secured to an input shaft and alsoconnecting a clutch of the hydraulic transmission gear to therebytransmit the power from the prime mover through the hydraulictransmission gear to the output shaft according to a signal transmittedby the control unit, and changing over the rotational direction of theoutput shaft by actuating a hydraulic valve provided in a fluid passageconnected between the hydraulic pump and the hydraulic motor and adaptedto be changed over according to a signal transmitted by the controlunit. When the rotational speed of the output shaft is higher than thepredetermined value, the method includes the steps of connecting theclutch of the mechanical transmission gear and also disconnecting theclutch of the hydraulic transmission gear to thereby transmit the powerfrom the prime mover through the mechanical transmission gear accordingto a command from the control unit, and minimizing the power required bythe hydraulic transmission gear according to a command from the controlunit.

To achieve the above-mentioned objects, according to a twelfth aspect ofthe present invention, there is provided a method of controlling powertransmission using a mechanical-hydraulic transmission gear system asset forth in the eleventh aspect, characterized in that to minimize thepower required by the hydraulic transmission gear, the discharge offluid from the hydraulic pump and that from the hydraulic motor arereduced to zero by actuating displacement control devices of the pumpand the motor according to a command from the control unit.

To achieve the above-mentioned objects, according to a thirteenth aspectof the present invention, there is provided a method of controllingpower transmission using a mechanical-hydraulic transmission gear as setforth in the eleventh aspect, characterized in that to minimize thepower required by the hydraulic transmission gear, the rotation of atleast one of the hydraulic pump and the hydraulic motor is stopped bymeans of the clutch thereof according to a command from the controlunit.

To achieve the above-mentioned objects, according to a fourteenth aspectof the present invention, there is provided a method of controllingpower transmission using a mechanical-hydraulic transmission gear systemas set forth in the eleventh aspect, when the hydraulic valve is locatedat its neutral position and vicinity, the fluid under pressure to besupplied to the hydraulic pump can be supplied to another fluid circuit.

To achieve the above-mentioned objects, according to a fifteenth aspectof the present invention, there is provided a method of controllingpower transmission using a mechanical-hydraulic transmission gear systemas set forth in the eleventh aspect, characterized in that irrespectiveof whether or not the hydraulic valve is operated, the fluid underpressure to be supplied to the hydraulic pump can be supplied to anotherfluid circuit by operating another manual fluid pressure change-overvalve provided in a passage connected between the hydraulic pump and thehydraulic motor.

To achieve the above-mentioned objects, according to a sixteenth aspectof the present invention, there is provided a method of controllingpower transmission using a mechanical-hyraulic transmission gear systemas set forth in the eleventh aspect, characterized in that at the stepof changing over the rotational direction of the output shaft byactuating the hydraulic valve, the rotational speed of the output shaftis controlled at the same time.

To achieve the above-mentioned objects, according to a seventeenthaspect of the present invention, there is provided amechanical-hydraulic transmission gear system for use in a constructionvehicle including a prime mover having a speed control unit; a hydraulictransmission gear having a variable displacement hydraulic pump and avariable displacement hydraulic motor; a control unit; and a mechanicaltransmission gear. The gear system further comprises a change-over meansfor changing forward running of the vehicle over to reversing and viceversa; a speed control means for controlling the running speed of thevehicle; a sensor for sensing the rotational speed of an output shaft;and a control means for comparing the signals transmitted by thechange-over means. The speed control means and the sensor means for theoutput shaft effect control of connection and disconnection of theclutch mounted on an input shaft of the mechanical transmission gear andeffect control of increase and decrease of the displacement of each ofthe pump and the motor.

To achieve the above-mentioned objects, according to an eighteenthaspect of the present invention, there is provided amechanical-hydraulic transmission gear system for use in a construcitonvehicle including a prime mover having a speed control unit; a hydraulictransmission gear having a variable displacement hydraulic pump and avariable displacement hydraulic motor; a control unit; and a mechanicaltransmission gear. The gear system further comprises a change-over meansfor changing forward running of the vehicle over to reversing and viceversa; a hydraulic valve provided in a fluid passage connected betweenthe pump and the motor and adapted to change over the rotationaldirection of an output shaft and also change over the flow of fluidunder pressure so as to flow into another fluid circuit according to asignal transmitted by the change-over means; a speed control means forcontrolling the running speed of the vehicle; a sensor means for sensingthe rotational speed of the output shaft; and a control means forcomparing the signals transmitted by the change-over means. The speedcontrol means and the sensor means for the output shaft effect controlof connection and disconnection of the clutch mounted on an input shaftof the mechanical transmission gear and effect control of increase anddecrease of the displacement of each of the pump and the motor.

To achieve the above-mentioned objects, according to a nineteenth aspectof the present invention, there is provided a mechanical-hydraulictransmission gear system for use in a construction vehicle including aprime mover having a speed control means; a hydraulic transmission gearhaving a variable displacement hydraulic pump and a variabledisplacement hydraulic motor; a control unit; and a mechanicaltransmission gear. The gear system further comprises a change-over meansfor changing forward running of the vehicle over to reversing and viceversa; a speed control means for controlling the running speed of thevehicle; a sensor means for sensing the rotational speed of an outputshaft; and a control means for comparing the signals transmitted by thechang-over means, the speed control means and the sensor means for theoutput shaft effect control of connection and disconnection of theclutches of the mechanical transmission gear and the hydraulictransmission gear and effect control of increase and decrease of thedisplacement of each of the pump and the motor.

To achieve the above-mentioned objects, according to a twentieth aspectof the present invention, there is provided a mechanical-hydraulictransmission gear system for use in a construction vehicle including aprime mover having a speed control means; a hydraulic transmission gearhaving a variable displacement hydraulic pump and a variabledisplacement hydraulic motor; a control unit; and a mechanicaltransmission gear. The gear system further comprising a change-overmeans for changing forward running of the vehicle over to reversing andvice versa; a hydraulic valve provided in a fluid passage connectedbetween the pump and the motor and adapted to change over the rotationaldirection of an output shaft and also change over the flow of fluidunder pressure so as to flow into another fluid circuit according to asignal transmitted by the change-over means; a speed control means forcontrolling the running speed of the vehicle; a sensor means for sensingthe rotational speed of the output shaft; and a control means forcomparing the signals transmitted by the change-over means. The speedcontrol means and the sensor means for the output shaft so as to effectcontrol of connection and disconnection of the clutches of themechanical transmission gear and the hydraulic transmission gear andeffect control of increase and decrease of the displacement of each ofthe pump and the motor.

To achieve the above-mentioned objects, according to a twenty-firstaspect of the present invention, there is provided amechanical-hydraulic transmission gear system for use in a constructionvehicle including a prime mover having a speed control means; ahydraulic transmission gear having a variable displacement hydraulicpump and a variable displacement hydraulic motor; a control unit; and amechanical transmission gear. The gear system further including achange-over means for changing forward running of the vehicle over toreversing and vice versa; a speed control means for controlling therunning speed of the vehicle; a sensor means for sensing the rotationalspeed of an output shaft; and a control means for comparing the signalstransmitted by the change-over means, the speed control means and thesensor means for the output shaft so as to effect control of connectionand disconnection of the clutches of the mechanical transmission gearand the hydraulic transmission gear and effect control of increase anddecrease of the displacement of each of the pump and/or the motor.

To achieve the above-mentioned objects, according to a twenty-firstaspect of the present invention, there is provided amechancial-hydraulic transmission gear system for use in a constructionvehicle including a prime mover having a speed control means; ahydraulic transmission gear having a variable displacement hydraulicpump and a variable displacement hydraulic motor; a control unit; and amechanical transmission gear. The gear system further comprising achange-over means for changing forward running of the vehicle over toreversing and vice versa; a first hydraulic valve provided in a fluidpassage connected between the pump and the motor and adapted to changeover the rotational direction of an output shaft and also control therotational speed of the output shaft according to a signal transmittedby the control unit; a second hydraulic valve provided in a fluidpassage connected between the pump and the motor and adapted to changeover the fluid under pressure to be supplied to the pump so as to flowinto another fluid circuit; a speed control means for controlling therunning speed of the vehicle; a sensor means for sensing the rotationalspeed of the output shaft; and a control means for comparing the signalstransmitted by the change-over means. The speed control means and thesensor means for the output shaft effect control of connection anddisconnection of the clutches of the mechanical transmission gear andthe hydraulic transmission gear and effect control of increase anddecrease of the displacement of the pump and/or the motor.

Principal advantages of the present invention having the above-mentionedaspects are as follows:

(1) During high speed rotation, only mechanical power transmission iseffected and the power loss due to fluid pressure is reducedsubstantially to zero.

(2) During low speed rotation, changes in the number of the revolutionsof output shaft from forward rotation to reversing and vice versa arecontrolled smoothly.

(3) The construction of the power transmission system is very simple andcan be manufactured at low cost, because it does not use the planetarygear system.

(4) The whole power transmission system can be made compact.

(5) In case the fluid discharged by the hydraulic pump is supplied intoanother actuator by changing over the hydraulic circuit, the energy offluid under pressure can be utilized without extra power losses.

(6) Change-over from the hydraulic transmission gear to the mechanicaltransmission gear and vice versa, and change-over in each of themechanical and hydraulic transmission gears can be carried out smoothlyby detecting the rotational speed of the output shaft, the degree ofopening of the throttle and changes in shift position and by controllingthe fluid pressure to be supplied to the clutches by means of thecontrol unit.

The above-mentioned and other objects, aspects and advantages of thepresent invention will become apparent to those skilled in the art bymaking reference to the following description and the accompanyingdrawings in which preferred embodiments incorporating the principles ofthe present invention are shown by way of example only.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are schematic overall configurational views showing amechanical-hydraulic transmission gear and a hydraulic transmissiongear, respectively, as prior art examples;

FIGS. 3 to 8 are schematic overall configurational views showing firstto sixth embodiments, respectively, of the present invention;

FIG. 9 is a flowchart showing the operation of the sixth embodiment ofthe present invention as shown in FIG. 8;

FIGS. 10A and 10B are graphs showing the relationship between therotational speed of the output shaft and the degree of opening of thethrottle of a prime mover, and the relationship between the rotationalspeed of the output shaft and the torque on the output shaft,respectively;

FIG. 11 is a table showing clutch engagement patterns;

FIGS. 12A to 12D are time charts, respectively, showing shift timing;

FIG. 13 is a flowchart showing hydraulic drive control operation;

FIGS. 14, 15, 16 and 17 are graphs showing the relationship between thedegree of opening of the throttle and the target rotational speed of theengine, the relationship between the degree of opening of the throttleand the target torque required by the hyraulic pump, the relationshipbetween the degree of opening of the throttle and the value of electriccurrent to be supplied to a hydraulic valve (or running control valve)and the relationship between the rotational speed of the output shaftand the discharge of fluid from the hydraulic motor, respectively.

FIG. 18 is a schematic overall configurational view showing the seventhembodiment of the present invention;

FIG. 19 is a flowchart showing the operation of the seventh embodimentof the present invention shown in FIG. 18;

FIGS. 20A to 20D are graphs showing the relationship between thepositions of the gear-shift lever and the rotational speed of the outputshaft, the degree of opening of the throttle, the torque on the outputshaft, and speed stages, respectively;

FIG. 21 is a graph showing the relationship between the rotational speedof the output shaft and the discharge of fluid from the hydraulic motor;

FIG. 22 is a graph showing the relationship between the rotational speedof the hydraulic motor and the discharge of fluid from the hydraulicmotor;

FIG. 23 is a table showing clutch engaging patterns; and

FIG. 24 is a tabular form showing combinations of a hydraulictransmission (H) and a mechanical transmission (M) at each of speedstages in the sixth and seventh embodiments of the present invention andexamples of application.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention will now be described by way of embodimentsthereof with reference to the accompanying drawings;

FIG. 3 is a schematic overall configurational view of amechanical-hydraulic gear system showing the first embodiment of thepresent invention, which comprises a prime mover 1 having a speedcontrol means, a hydraulic transmission gear 10, a control section 20,and a mechanical transmission gear 30. The hydraulic transmission gear10 comprises a variable displacement hydraulic pump 11 (which isreferred to simply as "pump" below) connected through an input shaft 2to the prime mover 1 so as to be driven by the latter; and a variabledisplacement hydraulic motor 13 (which is referred to simply as "motor"below) connected to the pump 11 by piping 12 and which is connected toan output shaft 14 adapted to be driven by the motive power developed bythe motor 13. The pump 11 and the motor 13 are provided with dischargecontrol valves 15 and 16, respectively, adapted to vary the discharge offluid from the pump 11 and the direction of flow of fluid dischargedthereby and the discharge of fluid from the motor 13 according to acommand from the control section 21. The input shaft 2 and the outputshaft 14 are provided with speed sensors 25 and 22, respectively, so asto feed back the number of revolutions thereof to the control section21. The mechanical transmission gear 30 comprises a clutch 31 fixedlysecured to the input shaft 2, a gear 32 rotatably mounted on the inputshaft 2, and a gear 33 meshing with the gear 32 and fixedly secured tothe output shaft 14, and is arranged such that it may be supplied withfluid under pressure from a pump 23 through a clutch pressurecontrolling valve 24 which is under control of the control section 21,thereby connecting the clutch 31 and rotating the output shaft 14. Thecontrol section 21 comprising a controller or control unit is providedwith a change-over means 26 such as a change lever for changing forwardrunning of a construction vehicle over to reversing and vice versa, anda speed control means 27 such as an accelerator pedal for controllingthe running speed of the vehicle, both of which constitute a controlunit 20.

The operation of the first embodiment having the above-mentionedconfiguration will be described below. When the rotational speed of themotor 13 or the output shaft 14 is lower than a predetermined value, thecontrol section 21 transmits a signal to the clutch pressure controllingvalve 24 so as to disconnect the clutch 31 of the mechanicaltransmission gear 30 fixedly secured to the input shaft 2, and thehydraulic transmission gear 10 transmits power to the output shaft 14 soas to drive a starting wheel 100 mounted on the body of a constructionvehicle, not shown. At that time, the speed control means 27 such as theaccelerator pedal transmits a signal to the control section 21 whichtransmits, in turn, a signal to the valve 15 for controlling thedischarge of fluid from the pump 11 and the valve 16 for controlling thedischarge of fluid from the motor 13 to thereby control the discharge offluid from the pump 11 and the motor 13, respectively, so that therotational speed of the motor 13 or the output shaft 14 can be set atwill without having to control the rotational speed of the primemover 1. Further, the change-over means 26 such as the change lever, etcfor changing forward running of the construction vehicle over toreversing and vice versa transmits a signal to the control section 21which transmits, in turn, a command signal to the pump 11 to thereby setthe discharge of fluid from the pump 11 in a negative zone to reversethe direction of fluid discharged by the pump 11 so that the rotationaldirection of the output shaft 14 can be reversed. Therefore, when therotational speed of the output shaft 14 is lower than a predeterminedvalue, the rotational speed of the output shaft during its forward andreverse rotation can be controlled smoothly. While, when the rotationalspeed of the motor 13 or the output shaft 14 is higher than apredetermined value and when the motor or the output shaft is rotatingforward or control of forward rotation is made, the control section 21transmits a signal to the clutch pressure controlling valve 24 so as toconnect the clutch 31 of the mechanical transmission gear 30 fixedlysecured to the input shaft 2 to thereby transmit the power through thegears 32 and 33 to the output shaft 14. At that time, the controlsection 21 transmits a command signal to the valve 15 for controllingthe discharge of fluid from pump 11 and the valve 16 for controlling thedischarge of fluid from motor 13 to actuate them, thereby reducing thedischarge of fluid from pump 11 and motor 13, respectively, to zero. Asa result, the power required by the pump 11 and the motor 13,respectively, is minimized, and the power developed by the prime mover 1is transmitted to the output shaft 14 only through the mechanicaltransmission gear 30 having an excellent power transmission efficiency.The speed control at that time is made by a speed control means, notshown, of the prime mover 1 adapted to be actuated by a signal which istranmitted by the control section 21 when it receives a signaltransmitted by the speed control means 27 such as the accelerator pedal,etc.

FIG. 4 is a schematic overall configurational view showing the secondembodiment of the present invention. In this drawing, the same componentparts as those in the above-mentioned first embodiment are denoted withthe same reference numerals and characters, and therefore thedescription thereof is omitted herein.

In this second embodiment, the power developed by a hydraulic motor 13of a hydraulic transmission gear 10a is transmitted to an output shaft14 through a gear 18 fixedly secured to a rotating shaft 111 of thehydraulic motor 13, a gear 19 rotatably mounted on the output shaft 14,and a clutch 17 fixedly secured to the output shaft 14. The clutch 17 issupplied with fluid under pressure discharged by a pump 23 driven by theprime mover 1 through a clutch pressure controlling valve 121 which isunder control of the control section 21, so that it is connected so asto rotate the output shaft 14.

The operation of the second embodiment having the above-mentionedconfiguration will be described below.

When the rotational speed of the motor 13 or the output shaft 14 islower than a predetermined value, the control section 21 transmits asignal to a clutch pressure controlling valve 24 and a clutch pressurecontrolling valve 121 so as to disconnect a clutch 31 of a mechanicaltransmission gear 30 and connect a clutch 17 of a hydraulic transmissiongear 10a to transmit the power from the prime mover 1 through thehydraulic transmission gear 10a to the output shaft 14, thereby drivinga starting wheel 100 mounted on the body of a construction vehicle, etcnot shown.

Setting of the rotational speed of the motor 13 or the output shaft 14at any desired value and change-over of the rotational direction of theoutput shaft 14 from forward to backward and vice versa can be made inthe same manner as that in the case of the first embodiment, andtherefore the description thereof is omitted herein.

In case the rotational speed of the pump 11 and the output shaft 14 arehigher than their respective predetermined values, and when the pump andthe output shaft are rotating forward or control of the forward rotationis made, the control section 21 transmits a signal to the clutchpressure controlling valve 24 of the clutch 31 and the clutch pressurecontrolling valve 121 of the clutch 17 so as to connect the clutch 31 ofthe mechanical transmission gear 30 and disconnect the clutch 17 of thehydraulic transmission gear 10a, thereby transmitting the power to theoutput shaft 14. At that time, the control section 21 transmits acommand signal to the valve 15 for controlling the discharge of fluidfrom the pump 11 to thereby actuate the valve 15 and reduce thedischarge of fluid from the pump 11 to zero.

In consequence, the power required by the pump 11 is minimized and thepower developed by the prime mover 1 is transmitted to the output shaft14 only through the mechanical transmission gear 30 having an excellentpower transmission efficiency. The speed control at that time is made bya speed control means, not shown, of the prime mover 1 adapted to beactuated by a signal which is transmitted by the control section 21 whenit receives a signal transmitted by the speed control means 27 such asthe accelerator pedal, etc.

FIG. 5 shows the third embodiment of the present invention. In thisdrawing, the same component parts as those in the first embodiment aredenoted with the same reference numerals and characters, and so thedescription thereof is omitted herein. In a hydraulic transmission gear110, a pump shaft 111 of a pump 11 connected through an input shaft 2 toa prime mover 1 so as to be driven by the latter is provided with aclutch 112. Further, the power developed by a motor 13 is transmitted toan output shaft 14 through a clutch 114 mounted on a motor shaft 113.Each of the clutches 112 and 114 is connected when it is supplied withfluid under pressure discharged by the pump 23 through the clutchpressure controlling valve 121 which is under control of the controlsection 21.

In the above-mentioned configuration, when the rotational speed of themotor 13 or the output shaft 14 is lower than a predetermined value, thecontrol section 21 transmits a command signal to a clutch pressurecontrolling valve 24 so as to disconnect a clutch 31 of a mechanicaltransmission gear 30 fixedly secured to the input shaft 2. In addition,the control section 21 transmits a signal to the clutch pressurecontrolling valve 121 so as to connect the clutches 112 and 114. As aresult, the power is transmitted by the hydraulic transmission gear 110to the output shaft 14 so as to drive a starting wheel 100 mounted onthe body of a construction vehicle, not shown.

In case the rotational speed of the pump 11 and the output shaft 14 arehigher than the respective predetermined values and when the pump andthe output shaft are rotating forward or control of the forward rotationis made, the control section 21 transmits a signal to the clutchpressure controlling valve 24 so as to connect the clutch 31 of themechanical transmission gear 30 fixedly secured to the input shaft 2,and also the control section 21 transmits a signal to the clutchpressure controlling valve 121 so as to disconnect the clutches 112,114, to thereby enable the power to be transmitted to the output shaft14 by the mechanical transmission gear 30. As a result, the pump 11 andthe motor 13 are not rotated, so that the power required by thehydraulic transmission gear can be reduced to zero and the powerdeveloped by the prime mover 1 can be transmitted to the output shaft 14only through the mechanical transmission gear 30 having an excellentpower transmission efficiency. While in this embodiment the clutches 112and 114 are mounted on the hydraulic transmission gear 110, they may bemounted on the mechanical transmission gear 30.

FIG. 6 shows the fourth embodiment of the present invention. In thisdrawing, the same component parts as those in the first embodiment aredenoted with the same reference numerals and characters and thereforethe description thereof is omitted herein. A hydraulic transmission gear200 comprises a one-side discharge type pump 11a connected through aninput shaft 201 to the prime mover 1 so as to be driven by the latter,and a motor 13 connected through a piping 202, a hydraulic valve 203 anda piping 204 to the pump 11a. The arrangement is made such that thepower developed by the motor 13 may be transmitted to an output shaft222 through a gear 221 for a speed increasing and decreasing device(speed multiplying/reducing gear set) 220 fixedly secured to a motorshaft 205, a gear 223 rotatably mounted on the output shaft 222 and aclutch 224 fixedly secured to the output shaft 222 or a clutch 225fixedly secured to a shaft 205, a gear 226 rotatably mounted on theshaft 205, and a gear 227 fixedly secured to the output shaft 222,thereby driving the output shaft 222. The pump 11a and the motor 13 areprovided with valves 15 and 16, respectively, for controlling thedischarge of fluid by a swash plate so that the discharge of fluid fromthe pump 11a and the discharge of fluid from the motor 13 may be changedaccording to a command from the control section 21. The hydraulic valve23 is provided with a port 203a for running the vehicle forward, a port203b for running the vehicle backward, and a port 203c for supplyingfluid to another hydraulic actuator (c). The clutches 224 and 225 aresupplied with fluid under pressure discharged by the pump 23 driven bythe prime mover 1 through clutch pressure controlling valves 228 and229, respectively, which are under control of the control section 21 soas to connect the clutch 224 or the clutch 225 to thereby rotate theoutput shaft 222. The output shaft 222 is provided with a speed sensor22 to feed back the number of revolutions thereof to the control section21. A mechanical transmission gear 250 comprises, in addition to thepower transmission elements such as the gears provided in the firstembodiment, a clutch 251 fixedly secured to the input shaft 201, a gear225 rotatably mounted on the input shaft 201, an intermediate gear 253,and a gear 254 fixedly secured to the shaft 205. In the mechancialtransmission gear 250, the fluid under pressure discharged by the pump23 is supplied either to the clutch 31 through the clutch pressurecontrolling valve 24 or to the clutch 251 through the clutch pressurecontrolling valve 255 under the control of the control section 21 so asto connect either the clutch 31 or the clutch 251 thereby rotating theshaft 205. The rotation of the shaft 205 is transmitted through thespeed increasing and reducing device 220 to the output shaft 222 tothereby rotate the same. The input shaft 201 is provided with a sensor25 so as to feed back the number of revolutions thereof to the controlsection 21. The control section 21 is provided with a change-over means26 for changing the forward running of a construction vehicle, etc overto reversing and vice versa and changing over the fluid to be suppliedto the pump to flow into another actuator such as an implement, and aspeed control means 27 such as accelerator pedal, etc for controllingthe running speed of the vehicle.

In the above-mentioned configuration, in case it is desired to run thevehicle forward or backward when the rotational speed of the motor 13 orthe output shaft 222 is lower than a predetermined value, thechange-over means 26 is changed over so as to send a signal to thecontrol section 21 which transmits, in turn, a signal for changing overthe hydraulic valve 203. Upon receipt of the signal, the hydraulic valve203 is changed over either to the port 203a for running the vehicleforward or to the port 203b for running the vehicle backward so as tochange over the direction of flow of the fluid under pressure from thepump 11a to the motor 13, thereby driving the vehicle forward orbackward. At that time, the clutches 31, 251 of the mechanicaltransmission gear 250 are disconnected, and either one of the clutches224, 225 of the speed increasing and decreasing device 220 is connectedaccording to a command from the control section 21 in such a way as tomatch the rotational speed of the output shaft 222 to thereby transmitthe power by the hydraulic transmission gear 200 to the output shaft222. The speed control of the output shaft 222 is made by a combinationof the discharge ratio between the pump 11a and the motor 13 and a gearratio between gears 221 and 223 or a gear ratio between gears 226 and227, all of which are stored in the control section 21 according to asignal transmitted by the speed control means 27 such as the acceleratorpedal, etc. When the change-over means 26 is not changed over and thehydraulic valve 203 is located at its neutral port 203c, power is nottransmitted to the output shaft 222 so that the fluid discharged by thepump 11a may be supplied to another hydraulic actuator, etc. Further, inthis embodiment, when the rotational speeds of the pump 11a and theoutput shaft 222 are higher than their respective predetermined values,either the clutch 31 or the clutch 251 of the mechanical transmissiongear 250 is connected irrespective of whether the output shaft 222 isrotating forward or backward, and also either the clutch 224 or theclutch 225 of the speed increasing and decreasing device 220 isconnected. Further, in the same manner as in the case of the firstembodiment, by setting the discharge of fluid from the pump 11a and themotor 13, respectively, at zero, the power developed by the prime mover1 is transmitted to the output shaft 222 only through the mechanicaltransmission gear 250 having an excellent power transmission efficiency.Further, which of the clutches 31, 251 is to be connected at that timeis decided according to a command from the control section 21 whichvaries depending on a signal from the speed control means 27 such asaccelerator pedal, etc and a signal transmitted by the change-over means26 for changing forward running of the vehicle over to reversing andvice versa.

Although in the above-mentioned embodiment the output shaft 222 isprovided with the speed sensor 22 to feed back the number of revolutionsthereof to the control section 21, the sensor may be provided on theshaft 205 of the motor 13.

FIG. 7 shows the fifth embodiment of the present invention. In thisdrawing, the same component parts thereof as those in the fourthembodiment are denoted with the same reference nuemrals and characters,and therefore the description thereof is omitted herein. A hydraulictransmission gear 300 comprises, in addition to the fourth embodimentincluding the pump 11a connected through the input shaft 201 to theprime mover 1 so as to driven by the latter and the motor 13 connectedthrough the piping 202, the hydraulic valve 203 and the piping 204 tothe pump 11a, a piping 301 branched from the piping 202 and a hydraulicvalve 302 connected to the piping 301 and which leads to anotherhydraulic actuator (D), to which fluid under pressure may be supplied orfrom which fluid may be returned, thereby driving the actuator. Thehydraulic valve 203 is provided with a port 203a for running the vehicleforward, a port 203b for running the vehicle backward, and a neutralport 203c. When the hydraulic valve 203 is located at neutral position,the fluid is returned to a fluid tank 303.

In the above-mentioned configuration, in case a vehicle such as aconstruction equipment is run forward or backward when the rotationalspeed of the motor 13 or the output shaft 14 is lower than apredetermined value, the change-over means 26 is changed over so as tosend a signal to the control section 21 which transmits, in turn, asignal for changing over the hydraulic valve 203. Upon receipt of thesignal, the hydraulic valve 203 is changed over either to the port 203afor running the vehicle forward or to the port 203b for running thevehicle backward so as to change the direction of flow of fluid from thepump 11a to the motor 13 thereby running the vehicle forward orbackward. In case it is desired to drive only another actuator (D), thechange-over means 26 is not changed over, and the hydraulic valve 302ais rendered operative to supply fluid discharged by the pump 11a eitherthrough the port 302a or through the port 302b to the hydraulic actuatorto be driven. Further, if the change-over means 206 for running thevehicle forward or backward is then operated, then the vehicle can bedriven forward or backward while another hydraulic actuator (D) is beingdriven. In the aforementioned fourth embodiment and the fifthembodiment, the input shaft 201 is directly connected to the pump 11aand the shaft 205 is directly connected to the motor 13, however, theshaft 201 may be connected through the clutch 112 to the pump 11a,whilst the shaft 205 may be connected through the clutch 114 to themotor 13 in the same manner as in the third embodiment. (Refer to FIG.5)

Further, in the above-mentioned first to fifth embodiments, thedirection of rotation of the output shaft is reversed by transmitting asignal from the change-over means such as the change-over lever forchanging forward running of the vehicle to backward running and viceversa to the control section, and transmitting a signal from the controlsection to the pump to thereby set the displacement of the pump in thenegative zone. However, such operation may be effected by setting thedisplacement of the motor in the negative zone instead. Further, a fixeddisplacement hydraulic motor may be provided in place of the variabledisplacement hydraulic motor. Moreover, in the above-mentionedembodiment, the gear ratio of the gears is set at one or two stages, butit may be set at multiple stages instead. Still further, in theembodiment shown, the piping to the clutches is branched, however it isneedless to say that a clutch pressure controlling valve may be providedfor each of the clutches.

FIG. 8 is a schematic overall configurational view showing the sixthembodiment of the present invention. In this drawing, the same componentparts as those in the above-mentioned first embodiment (Refer to FIG. 3)and those in the fifth embodiment (Refer to FIG. 7) are denoted with thesame reference numerals and characters, and therefore the descriptionthereof is omitted herein.

A hydraulic transmission type gear 400 comprises a hydraulictransmission gear 50 and a speed increasing and decreasing device 40.The hydraulic transmission gear 50 comprises a variable displacementhydraulic pump 11a (which is referred to simply as "pump 11a" below).The pump 11a is rotated by a pump shaft 3 which is driven through gears41, 42 of the speed increasing and decreasing device fixedly secured toa shaft 2 driven by the power developed by a prime mover 1. The variabledisplacement hydraulic motor 13 (which is referred to simply as "motor13" below) is connected to the pump 11a through a piping 301, ahydraulic valve 203 and a piping 204 so that it may receive fluid underpressure from the pump 11a and generate the power to be transmitted tothe motor shaft 4. The power transmitted to the motor shaft 4 istransmitted to the output shaft 14 through a clutch 43 of the speedincreasing and decreasing device 40 mounted on the motor shaft 4, a gear44 rotatably mounted on the motor shaft 4, and a gear 46 fixedly securedto the output shaft 14. When the clutch 43 is supplied with fluid underpressure discharged by a pump 23 through a clutch pressure controllingvalve 34 which is under control of a control unit 21, the clutch 43 isconnected, thereby rotating the output shaft 14.

The operation of the sixth embodiment having the above-mentionedconfiguration will be described below.

One example of the control operation is described with reference to theflowchart shown in FIG. 9. Further, for example "STEP 600" isabbreviated as "600". At 600, the degree of opening θt of the throttle,shift lever position Lp (R, N, 2nd, 1st), rotational speed No of theoutput shaft 14, and pressure P of fluid discharged by the pump 11a areread from the relevant sensors. At 601, it is discriminated whether ornot the shift lever position Lp is neutral position N and if Lp is N,then at 602 the data at the neutral position N are read. If Lp is not Nat 601, then at 603 it is discriminated whether the shift lever positionLp is a backward running position R or a first speed position 1st. Ifthe answer is No; that is, when the shift lever position Lp is a secondspeed position 2nd, then at 604 information on the shift is retrievedfrom data No, θt using a shift map showing shift patterns in FIGS. 10Aand 10B. At 605, a shift position command obtained from the result ofprocessing at 603 or 604 is compared with the current speed stage so asto discriminate whether the process should be proceeded either toshift-down zone or to shift-up zone. If at 605 there is no change inshift, then at 606 it is discriminated again whether the shift leverposition Lp is Ist or R. If, as a result, Lp is neither R nor 1st and is2nd, then the process is proceeded to STEP 612, and the result ofprevious processing or the initial value is outputted as it is. If at606 Lp is either R or 1st, then the process is proceeded to thehydraulic drive control subroutine at 607 which will be mentioned later.If at 605 there is a shift change, the system proceeds to 608 where aclutch engaging pattern as shown in FIG. 11 is read. The clutch engagingpattern is as shown in an example of shift timing data in FIG. 12, andcontrol electric currents Ica and Icb to be supplied from the controlsection 21 to the solenoids of the clutch pressure controlling valves 45and 24, respectively, are varied so as to connect or disconnect theclutches 43 and 31. Further, changes in control electric currents Icaand Icb at (Tf, Tt, Tm, etc.) against speed stages when a shift changeoccurs are given by the matrix as shown in FIG. 11. At 609, it isdetermined whether or not the shift change is a change from 1st to 2nd.In case the shift change is a speed change from 1st to 2nd, then at 611,as shown by an example of shift timing data in FIG. 12C, electriccurrent Ipd to be supplied from the control section 21 to the solenoidof the valve 15 for controlling the discharge of fluid from the pump 11aand electric current Ivf to be supplied from the control section 21 tothe solenoid of the hydraulic valve 203 are varied against thechange-over time t, and the resultant values are outputted at 612. If at609 the shift change is not a change from 1st to 2nd, then the processis proceeded to 610, where the start timing of hydraulic drive routineis read. The start timing is as shown by an example of shift timing datain FIG. 12D, and electric current Ipd to be supplied from the controlsection 21 to the solenoid of the valve 15 for controlling the dischargeof fluid from pump 11a, electric current Imd to be supplied from thecontrol section 21 to the solenoid of the valve 16 for controlling thedischarge of fluid from motor 13, and electric current Ivf to besupplied from the control section 21 to the solenoid of the hdyraulicvalve 203 are caused to rise after the start timing Ths. The hydraulicdrive control subroutine at STEP 600 which is outputted at 612 is asshown in a flowchart in FIG. 12. At step 700, a target number ofrevolutions Ne of the engine against the degree of opening θt of thethrottle obtained by depressing the accelerator pedal is calculated.(Refer to FIG. 13)

At step 701, deviation ei of the number of revolutions Ni of the shaft 2from the target number of revolutions Ne of engine is calculated. At702, the target torque Tpo required by the pump 11a against the degreeof opening θt of the throttle is calculated. (Refer to FIG. 15) At 703,the torque Tp required by the pump 11a when the deviation ei in therotational speed is taken into account is calculated, and at step 704the discharge Dp of fluid from pump 11a against the pressure P of fluiddischarge by the pump when the required torque Tp is obtained. At step705, control electric current Ipd commanded by the control section 21 tobe supplied to the solenoid of the discharge controlling valve 15 sothat the discharge Dp of fluid may be obtained in calculated. At step706, control electric current Iv commanded by the control section 21 tobe supplied to the solenoid of hydraulic valve 23 so as to obtain a flowrate of fluid matching the running speed of the vehicle against thedegree of opening θt of the throttle is calculated. (Refer to FIG. 16)At step 707, it is discriminated whether or not the shift lever positionLp is R. As a result, if Lp is not R, then at step 708, the controlelectric current Ivf for the forward running solenoid of the hydraulicvalve 203 is given the control electric current Iv which is calculatedat STEP 706, and the control electric current Ivr for the backwardrunning solenoid of the hydraulic valve 203 is given zero. Further, ifLp is R, then at step 709, control current Ivr for the backward runningsolenoid is given control current Iv which is calculated at step 706,whilst the control current Ivf is given zero. At step 710, the dischargeof fluid from the hydraulic motor 13 against the rotational speed No ofthe output shaft or the running speed of the vehicle is calculated.(Refer to FIG. 17) At step 711, the control current Imd for controllingthe hydraulic motor which enables the discharge of fluid calculated atstep 710 to be obtained is calculated.

FIG. 18 shows the seventh embodiment. In the drawing, the same componentparts thereof as those in the first and sixth embodiments are denotedwith the same reference numerals and characters, and therefore thedescription thereof is omitted herein.

The first speed stage 1st is formed in the same construction as that ofthe sixth embodiment. The second speed stage 2nd comprises a gear 48rotatably mounted on an output shaft 14 and which meshes with a gear 47fixedly secured to a motor shaft 4, and a clutch 49 of a speedincreasing and decreasing device 401 mounted on the output shaft 14.When the clutch 49 is supplied with fluid discharged by the pump 23through a clutch pressure controlling valve 150 which is under controlof the control section 21, the clutch 49 is connected so as to transmitthe power transmitted to the motor shaft 4 to the output shaft 14. Amechanical transmission gear 30a comprises a third speed stage 3rd and afourth speed stage 4th, which are formed in the same construction as thesecond speed stage 2nd of the sixth embodiment. The third speed stage3rd comprises a clutch 31 fixedly secured to a shaft 2 of the mechanicaltransmission gear 30a, a gear 32 rotatably mounted on the shaft 2, and agear 33 meshing with the gear 32 and fixedly secured to the output shaft14. When the clutch 31 is supplied with fluid under pressure dischargedby the pump 23 through a clutch pressure controlling valve 24 which isunder control of the control section 21, the clutch 31 is connected soas to transmit the power developed by the prime mover 1 to the outputshaft 14. The fourth speed stage 4th comprises likewise a clutch 34, agear 35, a gear 36, and a clutch pressure controlling valve 37. Further,in this case, the clutches 31, 34 and the gears 32, 35 may be providedon the output shaft 14; and the gears 33, 36 may be provided on theshaft 2, or they may be provided alternately.

The operation of the seventh embodiment having the above-mentionedconfiguration will be described below with reference to a flowchartshown in FIG. 19. In the case of this embodiment, the shift leverposition Lq includes R2 for backward running at second speed, R1 forbackward running at first speed, N for neutral position, and 1, 2, 3 andD for forward running at first to fourth speeds, respectively. At step804, information on gear shift is retrieved from data No and θt using ashift map showing shift patterns against shift lever position Lq inFIGS. 20A to 20D. At step 805, a shift position command obtained fromthe result of processing at 803 or 804 is compared with the currentspeed stage so as to discriminate whether the process is to be proceededto a shift down zone or to a shift up zone. If at step 805 there is noshift change, then at step 806 it is discriminated whether the currentspeed stage is at hydraulic drive positions 1st, 2nd, R 1st, R 2nd. Ifthe current speed stage is not at such positions, namely, it is at 3rdor 4th, then the process is proceeded to 813. If at step 806 the currentspeed stage is at either one of 1st, 2nd, R 1st, R 2nd, the process isproceeded to hydraulic drive control subroutine step 807, as in the caseof the sixth embodiment. At that time, at STEP 607 in the sixthembodiment and also at STEP 807 in seventh embodiment, the discharge offluid from the hydraulic motor against the rotational speed of theoutput shaft will change according to 1st, 2nd or R 1st, R 2nd, as shownin FIG. 21. At that time, as shown in FIG. 22, the rotational speed ofthe motor shaft 4 may be detected in place of the rotational speed ofthe output shaft. If at step 805 the occurrence of shift change isconfirmed, then the process is proceeded to step 808 where the clutchengaging pattern (Refer to FIG. 23) is read. The clutch engaging patternis as shown in an example of shift timing data in FIG. 12A, and eitherone of control currents Ica, Icb, Icc or Icd to be supplied from thecontrol section 21 to the solenoids of the clutch pressure controllingvalves 45, 150, 24 and 37 is varied so as to connect or disconnect oneof the clutches 43, 49, 31 or 34. Further, changes in the controlcurrents Ica, Icb, Icc aand Icd at (Tf, Tt, Tm, etc.) against the speedstages when a shift change occurs are made according to the matrix asshown in FIG. 23. At step 809, it is determined whether the shift changeis a change-over from hydraulic drive to direct connection of themechanical transmission gear 40a. As a result, if it is a change-overfrom hydraulic drive to direct connection, then at step 810, as shown byan example of shift timing data in FIG. 12C, electric current Ipd to besupplied by the control section 21 to the solenoid of the valve 15 forcontrolling the discharge of fluid from the pump 11a and electriccurrent Ivf to be supplied by the control section 21 to the solenoid ofthe hydraulic valve 203 are varied against the change-over time t, andsignals indicative of varied electric currents are outputted at step813. In case the shift change is not a change-over from hydraulic drivemode to mechanical transmission mode, then the process is proceeded tostep 811 where it is determined whether the shift change is achange-over from the mechanical transmission mode to the hydraulic drivemode. If not, the process is proceeded to step 813, and the result ofdiscrimination is outputted as it is. In case the shift change is achange-over from hydraulic drive to direct connection of the mechanicaltransmission gear 30a, then at step 812, as shown by an example of shifttiming data in FIG. 12D and as in the case of the sixth embodiment, theelectric current Ipd to be supplied by the control section 21 to thesolenoid of the valve 15 for controlling the discharge of fluid from thepump 11a, and the electric current Ivf to be supplied by the controlsection 21 to the solenoid of the hydraulic valve 203 are varied againstthe change-over time t, and signals indicative of varied electriccurrents are outputted at 813.

Further, the above-mentioned sixth embodiment has one speed stage forhydraulic drive mode and one speed stage for mechanical drive mode, andthe seventh embodiment has two speed stages for hydraulic drive mode andtwo speed stages for mechanical drive mode. Combinations as shown inFIG. 24 may also be used. Moreover, while in the above-mentionedembodiments, one or two pieces of hydraulic valves are provided betweenthe pump and the motor, a multiplicity of hydraulic valves may beprovided therebetween, and also, tandem, series, parallel circuits orcomposite circuits may be used as hydraulic circuits. Further, in theabove-mentioned embodiments, the hydraulic circuits for the hydraulictransmission gear have been described of open circuits only, however, itis needless to say that closed circuits may be used as well.

I claim:
 1. A method for controlling power transmission using amechanical-hydraulic transmission gear system including a prime moverhaving a speed control means; a hydraulic transmission gear operativelyconnectable to said prime mover and having a variable displacementhydraulic pump and a variable displacement hydraulic motor; a controlunit; and a mechanical transmission gear having a clutch and beingoperatively connectable to said prime mover, said control unit being incommunication with at least said clutch,wherein when the rotationalspeed of an output shaft is lower than a predetermined value, the methodincludes the steps of disconnecting said clutch of the mechanicaltransmission gear fixedly secured to an input shaft and transmitting thepower from the prime mover through the hydraulic transmission gear tothe output shaft according to a signal transmitted by said control unit;wherein when the rotational speed of the output shaft is higher than thepredetermined value, the method includes the steps of connecting theclutch of said mechanical transmission gear to thereby transmit thepower from the prime mover through the mechanical transmission gear tothe output shaft according to a signal from said control unit, andminimizing the power required by the hydraulic transmission gearaccording to a command from said control unit.
 2. A method ofcontrolling power transmission using a mechanical-hydraulic transmissiongear system according to claim 1, characterized in that to minimize thepower required by said hydraulic transmission gear the discharge offluid from said hydraulic pump and that from said hydraulic motor arereduced to zero by actuating displacement control devices of said pumpand said motor according to a command from said control unit.
 3. Amethod of controlling power transmission using a mechanical-hydraulictransmission gear system including a prime mover having a speed controlmeans; a hydraulic transmission gear, having a clutch, being operativelyconnectable to said prime mover, and having a variable displacementhydraulic pump and a variable displacement hydraulic motor; a controlunit; and a mechanical transmission gear having a clutch and beingoperatively connectable to said prime mover, said control unit being incommunication with at least said clutches,wherein when the rotationalspeed of an output shaft is lower than a predetermined value, the methodincludes the steps of disconnecting said clutch of the mechanicaltransmission gear fixedly secured to an input shaft and connecting saidclutch of the hydraulic transmission gear to thereby transmit the powerfrom the prime mover through the hydraulic transmission gear to theoutput shaft according to a signal transmitted by said control unit;wherein when the rotational speed of the output shaft is higher than thepredetermined value, the method includes the steps of connecting theclutch of said mechanical transmission gear and disconnecting the clutchof the hydraulic transmission gear to thereby transmit the power fromthe prime mover through the mechanical transmission gear to the outputshaft according to a signal transmitted by said control unit, andminimizing the power required by the hydraulic transmission gearaccording to a command from said control unit.
 4. A method ofcontrolling power transmission using a mechanical-hydraulic transmissiongear system according to claim 3, characterized in that to minimize thepower required by the hydraulic transmission gear the discharge of fluidfrom said hydraulic pump and that from said hydraulic motor are reducedto zero by actuating displacement control devices of said pump and saidmotor according to a command from said control unit.
 5. A method ofcontrolling power transmission using a mechanical-hydraulic transmissiongear system according to claim 3, characterized in that to minimize thepower required by said hydraulic transmission gear the rotation of atleast one of said hydraulic pump and said hydraulic motor is stopped bymeans of the clutch thereof according to a command from said controlunit.
 6. A method of controlling power transmission using amechanical-hydraulic transmission gear system including a prime moverhaving a speed control means; a hydraulic transmission gear operativelyconnectable to said prime mover, having a clutch and having a variabledisplacement hydraulic pump and a variable displacement hydraulic motorwith a hydraulic valve therebetween; a control unit; and a mechanicaltransmission gear having a clutch and being operatively connectable tosaid prime mover, said control unit being in communication with at leastsaid clutches,wherein when the rotational speed of an output shaft islower than a predetermined value, the method includes the steps ofdisconnecting said clutch of the mechanical transmission gear fixedlysecured to an input shaft and also connecting said clutch of thehydraulic transmission gear to thereby transmit the power from the primemover through the hydraulic transmission gear to the output shaftaccording to a signal transmitted by said control unit, and changingover the rotational direction of the output shaft by actuating saidhydraulic valve provided in a fluid passage connected between said pumpand said motor and adapted to be changed over according to a signaltransmitted by said control system; wherein when the rotational speed ofthe output shaft is higher than the predetermined value, the methodincludes the steps of connecting the clutch of said mechanicaltransmission gear and disconnecting the clutch of the hydraulictransmission gear to thereby transmit the power from the prime moverthrough the mechanical transmission gear to the output shaft accordingto a signal transmitted by said control unit, and minimizing the powerrequired by said hydraulic transmission gear according to a command fromsaid control unit.
 7. A method of controlling power transmission using amechanical-hydraulic transmission gear system according to claim 6,characterized in that when said hydraulic valve is located at itsneutral position and vicinity the fluid under pressure to be supplied tosaid hydraulic pump can be supplied to another fluid circuit.
 8. Amethod of controlling power transmission using a mechanical-hydraulictransmission gear system according to claim 6, characterized in thatirrespective of whether or not said hydraulic valve is operated thefluid under pressure to be supplied to said hydraulic pump can besupplied to another fluid circuit by operating another manual fluidpressure change-over valve provided in a fluid passage connected betweensaid pump and said motor.
 9. A method of controlling power transmissionusing a mechanical-hydraulic transmission gear system including a primemover having a speed control means; a hydraulic transmission gearoperatively connectable to said prime mover and having a variabledisplacement hydraulic pump and a variable displacement hydraulic motor;a control unit; a mechanical transmission gear having a clutch and beingoperatively connectable to said prime mover, said control unit being incommunication with at least said clutch; and a speedmultiplying/reducing gear set operatively connectable to said hydraulictransmission gear and said mechanical transmission gear,wherein when therotational speed of an output shaft is lower than a predetermined value,the method includes the steps of disconnecting said clutch of themechanical transmission gear fixedly secured to an input shaft andtransmitting the power from the prime mover through said hydraulictransmission gear and said speed multiplying/reducing gear set to theoutput shaft according to a signal transmitted by said control unit;wherein when the rotational speed of the output shaft is higher than thepredetermined value, the method includes the steps of connecting theclutch of said mechanical transmission gear to thereby transmit thepower from the prime mover through the mechanical transmission gear andsaid speed multiplying/reducing gear set to the output shaft accordingto a signal transmitted by said control unit, and minimizing the powerrequired by said hydraulic transmission gear according to a command fromsaid control unit.
 10. A method of controlling power transmission usinga mechanical-hydraulic transmission gear system according to claim 9,characterized in that to minimize the power required by said hydraulictransmission gear the discharge of fluid from said hydraulic pump andthat from said hydraulic motor are reduced to zero by actuatingdisplacement control devices of said pump and said motor according to acommand from said control unit.
 11. A method of controlling powertransmission using a mechanical-hydraulic transmission gear systemincluding a prime mover having a speed control means; a hydraulictransmission gear operatively connectable to said prime mover, having aclutch, and having a variable displacement hydraulic pump and a variabledisplacement hydraulic motor with a hydraulic valve therebetween; acontrol unit; a mechanical transmission gear having a clutch and beingoperatively connectable to said prime mover, said control unit being incommunication with at least said clutches; and a speedmultiplying/reducing gear set,wherein when the rotational speed of anoutput shaft is lower than a predetermined value, the method includesthe steps of disconnecting a clutch of the mechanical transmission gearfixedly connected to an input shaft and also connecting a clutch of thehydraulic transmission gear to thereby transmit the power from the primemover through the hydraulic transmission gear to the output shaftaccording to a signal transmitted by said control unit, and changingover the rotational direction of the output shaft by actuating ahydraulic valve provided in a fluid passage connected between saidhydraulic pump and said hydraulic motor and adapted to be changed overaccording to a signal transmitted by said control unit; wherein when therotational speed of the output shaft is higher than the predeterminedvalue, the method includes the steps of connecting the clutch of saidmechanical transmission gear and also disconnecting the clutch of saidhydraulic transmission gear to thereby transmit the power from the primemover through the mechanical transmission gear to the output shaftaccording to a command from said control unit, and minimizing the powerrequired by said hydraulic transmission gear according to a command fromsaid control unit.
 12. A method of controlling power transmission usinga mechanical-hydraulic transmission gear system according to claim 11,characterized in that to minimize the power required by said hydraulictransmission gear the discharge of fluid from said hydraulic pump andthat from said hydraulic motor are reduced to zero by actuatingdisplacement control devices of said pump and said motor according to acommand from said control unit.
 13. A method of controlling powertransmission using a mechanical-hydraulic transmission gear systemaccording to claim 11, characterized in that to minimize the powerrequired by said hydraulic transmission gear the rotation of at leastone of said hydraulic pump and said hydraulic motor is stopped by meansof the clutch thereof according to a command from said control unit. 14.A method of controlling power transmission using a mechanical-hydraulictransmission gear system according to claim 11, characterized in thatwhen said hydraulic valve is located at its neutral position andvicinity the fluid under pressure to be supplied to said hydraulic pumpcan be supplied to another fluid circuit.
 15. A method of controllingpower transmission using a mechanical-hydraulic transmission gear systemaccording to claim 11, characterized in that irrespective of whether ornot said hydraulic valve is operated the fluid under pressure to besupplied to said hydraulic pump can be supplied to another fluid circuitby operating another manual fluid pressure change-over valve provided ina fluid passage connected between said hydraulic pump and said hydraulicmotor.
 16. A method of controlling power transmission using amechanical-hydraulic transmission gear system according to claim 11,characterized in that at the step of changing over the rotationaldirection of the output shaft by actuating said hydraulic valve therotational speed of the output shaft is controlled at the same time. 17.A mechanical-hydraulic transmission gear system for use in aconstruction vehicle including a prime mover having a speed controlmeans; a hydraulic transmission gear operatively connectable to saidprime mover and having a variable displacement hydraulic pump and avariable displacement hydraulic motor; a control unit; and a mechanicaltransmission gear having a clutch and being operatively connectable tosaid prime mover, said control unit in communication with at least saidclutch and comprising a control means, the gear system furthercomprisinga change-over means for changing forward running of thevehicle over to reversing and vice versa; a speed control means forcontrolling the running speed of the vehicle; and a sensor means forsensing the rotational speed of an output shaft; wherein said controlmeans is in communication with and is for comparing the signalstransmitted by said change-over means, said speed control means and saidsensor means for said output shaft so as to effect control of connectionand disconnection of the clutch mounted on an input shaft of themechanical transmission gear and effect control of increase and decreaseof the displacement of each of said pump and said motor.
 18. Amechanical-hydraulic transmission gear system for use in a constructionvehicle including a prime mover having a speed control means; ahydraulic transmission gear operatively connectable to said prime moverand having a variable displacement hydraulic pump and a variabledisplacement hydraulic motor with a hydraulic valve therebetween; acontrol unit; and a mechanical transmission gear having a clutch andbeing operatively connectable to said prime mover, said control unit incommunication with at least said clutch and comprising a control means,the gear system further comprisinga change-over means for changingforward running of the vehicle over to reversing and vice versa; saidhydraulic valve provided in a fluid passage connected between said pumpand said motor and adapted to change over the rotational direction of anoutput shaft and also change over the flow of fluid under pressure so asto flow into another fluid circuit according to a signal transmitted bysaid change-over means; a speed control means for controlling therunning speed of the vehicle; and a sensor means for sensing therotational speed of an output shaft; wherein said control means is incommunication with and is for comparing the signals transmitted by saidchange-over means, said speed control means and said sensor means forsaid output shaft so as to effect control of connection anddisconnection of the clutch mounted on an input shaft of the mechanicaltransmission gear and effect control of increase and decrease of thedisplacement of each of said pump and said motor.
 19. Amechanical-hydraulic transmission gear system for use in a constructionvehicle including a prime mover having a speed control means; ahydraulic transmission gear operatively connectable to said prime mover,having a clutch, and having a variable displacement hydraulic pump and avariable displacement hydraulic motor; a control unit; and a mechanicaltransmission gear operatively connectable to said prime mover and havinga clutch, said control unit in communication with at least said clutchesand comprising a control means, the gear system further comprisingachange-over means for changing forward running of the vehicle over toreversing and vice-versa; a speed control means for controlling therunning speed of the vehicle; and a sensor means for sensing therotational speed of an output shaft; wherein said control means is incommunication with and is for comparing the signals transmitted by saidchange-over means, said speed control means and said sensor means forsaid output shaft so as to effect control of connection anddisconnection of the clutches of the mechanical transmission gear andthe hydraulic transmission gear and effect control of increase anddecrease of the displacement of each of said pump and said motor.
 20. Amechanical-hydraulic transmission gear system for use in a constructionvehicle including a prime mover having a speed control means; ahydraulic transmission gear operatively connectable to said prime mover,having a clutch, and having a variable displacement hydraulic pump and avariable displacement hydraulic motor with a hydraulic valvetherebetween; a control unit; and a mechanical transmission gear havinga clutch and being operatively connectable to said prime mover, saidcontrol unit in communication with at least said clutch and comprising acontrol means, the gear system further comprisinga change-over means forchanging forward running of the vehicle over to reversing and viceversa; said hydraulic valve provided in a fluid passage connectedbetween said pump and said motor and adapted to change over therotational direction of an output shaft and change over the flow offluid under pressure so as to supply it to another fluid circuitaccording to a signal transmitted by said change-over means; a speedcontrol means for controlling the running speed of the vehicle; and asensor means for sensing the rotational speed of the output shaft;wherein said control means is in communication with and is for comparingthe signals transmitted by said change-over means, said speed controlmeans and said sensor means for said output shaft so as to effectcontrol of connection and disconnection of the clutches of themechanical transmission gear and the hydraulic transmission gear andeffect control of increase and decrease of the displacement of each ofsaid pump and said motor.
 21. A mechanical-hydraulic transmission gearsystem for use in a construction vehicle including a prime mover havinga speed control means; a hydraulic transmission gear operativelyconnectable to said prime mover, having a clutch, and having a variabledisplacement hydraulic pump and a variable displacement hydraulic motor;a control unit; and a mechanical transmission gear having a clutch andbeing operatively connectable to said prime mover, said control unit incommunication with at least said clutches and comprising control means,the gear system further comprisinga change-over means for changingforward running of the vehicle over to reversing and vice versa; a speedcontrol means for controlling the running speed of the vehicle; and asensor means for sensing the rotational speed of an output shaft;wherein said control means is in communication with and is for comparingthe signals transmitted by said change-over means, said speed controlmeans and said sensor means for said output shaft so as to effectcontrol of connection and disconnection of the clutches of themechanical transmission gear and the hydraulic transmission gear andeffect control of increase and decrease of the displacement of at leastone of said pump and said motor.
 22. A mechanical-hydraulic transmissiongear system for use in a construction vehicle including a prime moverhaving a speed control means; a hydraulic transmission gear operativelyconnectable to said prime mover and having a variable displacementhydraulic pump and a variable displacement hydraulic motor with firstand second hydraulic valves therebetween; a control unit; and amechanical transmission gear having a clutch and being operativelyconnectable to said prime mover, said control unit in communication withat least said clutches and comprising a control means, the gear systemfurther comprisinga change-over means for changing forward running ofthe vehicle over to reversing and vice versa; said first hydraulic valveprovided in a fluid passage connected between said pump and said motorand adapted to change over the rotational direction of an output shaftand control the rotational speed of the output shaft according to asignal transmitted by said control unit; said second hydraulic valveprovided in said fluid passage connected between said pump and saidmotor and adapted to change over the fluid under pressure to be suppliedto the pump so as to flow into another fluid circuit; a speed controlmeans for controlling the running speed of the vehicle; a sensor meansfor sensing the rotational speed of the output shaft; wherein saidcontrol means is in communication with and is for comparing the signalstransmitted by said change-over means, said speed control means and saidsensor means for said output shaft for controlling disconnection andconnection of the clutches of the mechanical transmission gear and thehydraulic transmission gear and for controlling the increase anddecrease of the displacement of at least one of said pump and saidmotor.